The WHO reported 368,207 suspected cases and 1,828 deaths in the Arabian Peninsula country since late April.

Surveillance data "confirms a slight decline in suspected cases over the past two weeks" in some of the most affected governorates - Amanat Al Asimah, Amran and Sanaa, Chaib said.

But great caution was called for as there is a backlog in reporting and data is still being analysed, she said.

Oxfam, an international charity based in Britain, said: "It is feared that the total number of people infected could eventually rise to over 600,000, making it one of the largest outbreaks since records began in 1949."

The WHO did not provide its latest planning figure.

Rupert Colville, UN human rights spokesman, said on Friday that the fighting between the government forces - backed by Saudi Arabia-led coalition - and the Houthi rebels was "hugely exacerbating the ability to stop this epidemic of cholera".

"The kind of disintegration of the health system in Yemen as a result of the conflict at a time of cholera is an absolutely lethal combination," said Colville.

Oxfam's Kjetil Ostnor said a massive aid was needed for dealing with the crisis, but first of all, a ceasefire must be secured.

"We need access to the whole country. We need to be able to travel in and out of the country and around the country," Ostnor said.

The war has already killed about 10,000 people since it broke out in 2014. At least 15 million people are without safe drinking water or hygienic sanitation. Seven million Yemenis are malnourished.